The fact that there is a competitive candidate replacement native in Julia (Although a few man years in engineering time wouldn't hurt) is telling.
"written in pure python" is just moving the goalposts.
There's no need for them to be "written in pure python". They just need to be seemlessly usable from within it.
Your argument is that “Python can call fast code written in other languages”? That is true, of course, but if that’s your definition of Python being fast, then it makes the notion of one language being faster than another meaningless.